1. Field of the Invention
The present invention relates to a method of and apparatus for cleaning a cylinder and a roller of a printing machine. More particularly, the present invention is concerned with a method of and apparatus for cleaning a cylinder of an offset printing machine by pressing a cleaning cloth onto the outer peripheral surface of the blanket by means of a pressure pad. Still more particularly, the present invention pertains to a method of and apparatus for backward feeding the cleaning cloth in a cleaning system of the type mentioned above.
2. Description of the Prior Art
A typical known printing cylinder cleaning apparatus will be described hereinunder. Although the following description is concentrated specifically on an apparatus for cleaning a blanket cylinder of an offset printing machine, it is to be understood that the same printing cylinder cleaning apparatus can be used for cleaning other types of cylinders.
An offset printing machine usually has three cylinders, namely, a plate cylinder, a blanket cylinder and an impression cylinder. These cylinders are arranged such that their axes extend in parallel with one another and such that they can be brought into mutual contact. A printing plate made of, for example, a type alloy or aluminum is wound on the plate cylinder, while a sheet-like blanket such as of rubber is wound on the blanket cylinder. The printing plate has a grained surface and is provided with a water repellent layer carrying an image of characters or a picture. The surface of the printing plate other than portions having images of characters or picture is dampened as water is supplied to the surface of the printing plate by means of dampening device. An oily ink is applied to the surface of the printing plate by means of an inking device. The ink will attach to the portion of the plate surface carrying the image because this area is not dampened, but will be repelled by other portions of the plate surface due to the water content held by the grained surface. The ink thus held on the plate cylinder is transferred to the blanket cylinder and is further transferred to a printing paper which passes through the nip between the blanket cylinder and the impression cylinder. This is the principle of offset printing.
As the offset printing machine operates long, the blanket surface is contaminated due to accumulation of residual ink.
In order to obviate this problem, an apparatus has been developed which is capable of cleaning the blanket cylinder.
This apparatus has a cleaning cloth supply roll and a cleaning cloth take-up roll arranged in a pair on a pair of side plates which are mounted on the frame of the printing machine. A continuous cleaning cloth is wound at its both ends on these rolls and are suitably tensed between these rolls. The cleaning cloth take-up roll is driven by a driving device to rotate at a predetermined speed and intermittently, e.g., once every 3 seconds. A stay having a substantially T-shaped cross-section is provided to extend in the direction of axes of these rolls. Both ends of the stay are fixed to the adjacent side plates. The portion of the stay opposing to the blanket cylinder is made hollow, and the surface facing the blanket cylinder is hermetically lined with a pressure pad made of an elastic member. A plenum chamber which is defined by the end of the stay and the pressure pad is communicated with an external air compressor. As the air compressor operates, compressed air is supplied to the plenum chamber so that the pressure pad is expanded, whereby the cleaning cloth sliding on the outer surface of the pressure pad is pressed against the blanket cylinder so as to wipe off contaminant on the blanket wound on the blanket cylinder thereby cleaning the blanket surface. The pressure pad is made of an elastic resin or a rubber so that it elastically presses the cleaning cloth onto the surface of the blanket cylinder.
In a standard sheet offset printing machine, the cleaning cloth is fed intermittently, e.g., once for every three rotations of the blanket cylinder, by about 5 mm in each feeding cycle. Each cleaning cycle usually has, for example, 20 cycles of feed on the cleaning cloth. This means that about 100 mm of the cleaning cloth is consumed in each cycle of cleaning operation. In case of a newspaper offset printing press, about 450 mm of cleaning cloth is consumed in each cleaning cycle, though detailed numerical data is not shown.
The cleaning cloth has to meet various requirements such as high tensile strength, dimensional precision and wettability and, therefore, is usually made of an expensive material such as a non-woven fabric. This undesirably raises the running cost of the printing machine.